1. Field of the Invention
The present invention relates to a toner for developing electrostatic latent images used in image forming methods such as electrophotographic methods and electrostatic printing methods.
2. Description of the Related Art
As computers and multimedia become more advanced, there arises an increasing need to develop ways to output high-definition full color images that satisfy various needs in homes and offices.
In offices where large quantities of copies and printouts are made, image forming apparatuses desirably have high durability whereby degradation of image quality is suppressed even when a large number of copies and printouts are made.
In contrast, in small offices and homes, image forming apparatuses are desirably capable of producing high-quality images and are desirably small to save space and energy and reduce weight. To satisfy these needs, toners used therein desirably have improved properties, such as environmental stability, low-temperature fixability, development durability, and storage stability.
In particular, a full color image is formed by superimposing color toners. Unless all of the color toners are developed equally, the color reproducibility is degraded and color nonuniformity is generated. If a pigment or a dye used as a colorant of a toner is precipitated on the surfaces of toner particles, the developing performance is affected and color nonuniformity may result.
In forming a full color image, fixability and color mixing property during fixing are important. For example, in order to achieve high-speed image formation, a binder resin suitable for low temperature fixing is selected. The influence of this binder resin on the developing performance and durability is also large.
Moreover, a toner that can be used for a long time and is capable of forming high-definition full color images at various temperatures and humidity is in demand. In order to fulfill such need, several challenges are desirably addressed, such as suppressing changes in the toner surface properties and changes in the charge amount of toners caused by changes in the operation environment and minimizing soiling of parts such as a developing roller, a charging roller, a regulating blade, and a photosensitive drum. In this respect, development of a toner that exhibits stable chargeability despite being stored in a wide variety of environments for a long time and has stable development durability that does not cause soiling of parts has been eagerly anticipated.
One of the causes of changes in charge amount and storage stability of the toner due to temperature and humidity is a phenomenon called bleeding in which a release agent and a resin component in the toner ooze out from the interior of the toner particle to the surface of the toner particle, thereby altering the surface properties of the toner.
One way to address this challenge is to cover the surface of a toner particle with a resin.
Japanese Patent Laid-Open No. 2006-146056 discloses a toner that has good high-temperature storage stability and exhibits good printing durability when printing is conducted in a normal temperature, normal humidity environment or a high temperature, high humidity environment. This toner includes inorganic fine particles strongly fixed to toner particle surfaces.
However, even if inorganic fine particles are strongly fixed to toner particles, bleeding of a release agent and a resin component occurs through gaps between the inorganic fine particles and the inorganic fine particles may detach due to deterioration of durability. Accordingly, the durability in a severe environment is desirably further improved and the problem of soiling of parts is desirably addressed.
Japanese Patent Laid-Open No. 03-089361 discloses a method for producing a polymerized toner, in which a silane coupling agent is added to the reaction system to try to prevent colorants and polar substances from becoming exposed in the toner particle surfaces and to obtain a toner that has a narrow charge amount distribution and very low dependence of charge amount on humidity.
However, according to this method, the amount of precipitates of the silane compounds on the toner particle surfaces and hydrolytic polycondensation are insufficient. The environmental stability and the development durability are desirably further improved.
Japanese Patent Laid-Open No. 08-095284 discloses a way of controlling the charge amount of the toner and forming high-quality printed images irrespective of temperature and humidity in the environment. In particular, it discloses a polymerized toner in which a silane is used to coat surfaces of toner particles.
However, the polarity of organic functional groups is high and hydrolytic polycondensation and the amount of precipitates of the silane compound on the toner particle surfaces are insufficient. Further improvements are desired in order to enhance the storage stability, suppress soling of parts by toner fusion, and decrease the change in image density caused by changes in chargeability at high temperature and high humidity.
Japanese Patent Laid-Open No. 2001-75304 discloses a toner that improves fluidity, low temperature fixability, and blocking property and suppresses detachment of a fluidizer. This toner is a polymerized toner that includes a coating layer in which granular lumps containing a silicon compound are fixed to each other.
However, bleeding of a release agent and a resin component occurs through gaps between the granular lumps containing a silicon compound. The image density changes due to changes in chargeability in a high temperature, high humidity environment due to insufficient hydrolytic polycondensation and an insufficient amount of silane compound precipitates on the toner particle surfaces. Moreover, parts become soiled by toner fusion. These problems are desirably addressed and the storage stability is desirably further improved.